Use BJT transistor as a switch without inverting the signal

Question

I know I can use a transistor as a switch like this:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Can I use only one transistor that will output the same signal (not inverted)? I want to use it to shift voltage level. The input is digital at 3.3V and should output 5V TTL levels.

score 21 · Answer 1 · answered Apr 21 '14 at 17:14

21

Absolutely.

enter image description here

This circuit takes advantages of the properties of a MOSFET to bidirectionally switch a signal between two different voltage levels.

answered Apr 21 '14 at 17:14

Ignacio Vazquez-Abrams

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I was interested in doing this with a bipolar PNP or NPN. – Cornelius Apr 21 '14 at 17:15
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@Cornelius This answer is the better one dude because it is bidirectional whereas my circuit is not. – Andy aka Apr 21 '14 at 17:16
I know it's bidirectional. I have seen this circuit before in a schematic for an I2C level shifter. But for what I need unidirectional is enough. – Cornelius Apr 21 '14 at 17:19
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@Andyaka: You're a good guy. Ignacio: useful circuit :) – bitsmack Apr 21 '14 at 19:26
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@bitsmack yeah I'm all-heart muhuhahaha – Andy aka Apr 21 '14 at 20:05
If you have 5V on both sides, can you use single resistor? – DikobrAz Aug 07 '15 at 17:11
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If you have 5V on both sides then why do you need a level shifter? – Ignacio Vazquez-Abrams Aug 07 '15 at 18:51

score 20 · Accepted Answer · answered Apr 21 '14 at 17:14

20

This circuit tends to be used quite often: -

enter image description here

answered Apr 21 '14 at 17:14

Andy aka

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Very interesting. Would it be possible to put one GPIO of a board such Raspberry or BeagleBone at the input without frying the board? What I don't understand is: at the basis you have a constant voltage, the collector is pulled up. So how can you be sure that no current is moving from the 5V to the input? Eventually destroying the driving board? – Dave Jan 23 '16 at 20:37
Because the transistor is configured as an emitter follower and the voltage on the emitter is base voltage minus about 0.6V. If the emitter got higher it would turn off the transistor thus preventing the voltage rising much above about 3V. Think of base and emitter and what differential voltage they must be at to stat to turn the transistor on. – Andy aka Jan 23 '16 at 21:41
@Andyaka Will this work if we change 5V to 3.3V? – killdaclick Aug 14 '18 at 18:36
If you did that you wouldn’t need a level shifter. – Andy aka Aug 14 '18 at 21:22

The Photon · Answer 3 · 2014-04-21T17:53:56.703

The input is digital at 3.3V and should output 5V TTL levels.

For this situation you very likely do not need any conversion circuit at all.

Both 3.3 V and 5 V TTL logic switch with a threshold of about 0.8 V. Therefore no conversion circuit is needed to drive a 5 V TTL input with a 3.3 V logic signal. To be absolutely sure, check the minimum V_ih of your 5 V TTL device and make sure this number is less than the minimum V_oh of your 3.3 V device.

This figure, from the TI app note, "Selecting the Right Level-Translation Solution", shows that the specified input, output, and threshold levels are the same for 5 V and 3.3 V TTL circuits:

enter image description here

If your devices are actually not TTL but CMOS, you may still be able to connect them without a conversion circuit, but noise margin will be reduced because the CMOS switching threshold is V_cc/2 rather than a fixed voltage level.

As you see from the chart, you cannot reliably use a 3.3 V TTL signal (or even 5 V TTL) signal to switch a 5 V CMOS input, so for either of those situations you would need a converter circuit as outlined by the other answers here.

Use BJT transistor as a switch without inverting the signal

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